1. Field of the Invention
The present invention relates generally to nuclear reactors, and more particularly, is concerned with an improved control rod for use with a nuclear fuel assembly in reactor shutdown.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of tranverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. One common way of doing this is by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. Representative of the prior art are the control rods and systems disclosed in U.S. Pats. Nos. to Busby et al (3,088,898), Hitchcock (3,230,147 and 3,255,086), Eich (3,485,717), French et al (3,519,535), Schabert et al (3,734,825), Radkowsky et al (4,123,328), Bevilacqua (4,169,759) and Anthony et al (4,172,762).
One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill and assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly, which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. Generally, the neutron absorbing material is one having a high neutron absorption cross section, such as boron carbide, tantalum, a combination of silver-indium-cadmium, or many others well known in the art. The material is ordinarily in the form of a stack of closely packed ceramic or metal pellets which only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug which defines a plenum chamber for receiving gasses generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rod.
At end of cycle life (EOL) and hot zero power (HZP) core condition in reactors, an adverse power (flux) distribution shift to the top of the core commonly occurs. For instance, the axial flux imbalance (AFI) for a typical pressurized water reactor is on the order of 50 to 60%. With such extreme AFI, there is significant reactivity redistribution which nuclear designers have to account for in accident analysis via a reactivity penalty on available rod worth. The penalty, commonly known as reactivity redistribution factor (RRF), is approximately 0.85% delta p at EOL and accounted for in the shutdown margin calculation.
The control rod designs used heretofore have failed to adequately alter this imbalance in the core axial power distribution at reactor shutdown. Consequently, a need exists for a control rod design which will conteract this situation so as to substantially reduce RRF during reactor shutdown.